1 files changed, 235 insertions, 0 deletions
diff --git a/mlir/lib/Transforms/LoopUnrollAndJam.cpp b/mlir/lib/Transforms/LoopUnrollAndJam.cpp
new file mode 100644
index 00000000000..6c74d545497
--- /dev/null
+++ b/mlir/lib/Transforms/LoopUnrollAndJam.cpp
@@ -0,0 +1,235 @@
+//===- LoopUnrollAndJam.cpp - Code to perform loop unroll and jam ---------===//
+//
+// Part of the MLIR Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// This file implements loop unroll and jam. Unroll and jam is a transformation
+// that improves locality, in particular, register reuse, while also improving
+// operation level parallelism. The example below shows what it does in nearly
+// the general case. Loop unroll and jam currently works if the bounds of the
+// loops inner to the loop being unroll-jammed do not depend on the latter.
+//
+// Before      After unroll and jam of i by factor 2:
+//
+//             for i, step = 2
+// for i         S1(i);
+//   S1;         S2(i);
+//   S2;         S1(i+1);
+//   for j       S2(i+1);
+//     S3;       for j
+//     S4;         S3(i, j);
+//   S5;           S4(i, j);
+//   S6;           S3(i+1, j)
+//                 S4(i+1, j)
+//               S5(i);
+//               S6(i);
+//               S5(i+1);
+//               S6(i+1);
+//
+// Note: 'if/else' blocks are not jammed. So, if there are loops inside if
+// op's, bodies of those loops will not be jammed.
+//===----------------------------------------------------------------------===//
+#include "mlir/Transforms/Passes.h"
+
+#include "mlir/Analysis/LoopAnalysis.h"
+#include "mlir/Dialect/AffineOps/AffineOps.h"
+#include "mlir/IR/AffineExpr.h"
+#include "mlir/IR/AffineMap.h"
+#include "mlir/IR/BlockAndValueMapping.h"
+#include "mlir/IR/Builders.h"
+#include "mlir/Pass/Pass.h"
+#include "mlir/Transforms/LoopUtils.h"
+#include "llvm/ADT/DenseMap.h"
+#include "llvm/Support/CommandLine.h"
+
+using namespace mlir;
+
+#define DEBUG_TYPE "affine-loop-unroll-jam"
+
+static llvm::cl::OptionCategory clOptionsCategory(DEBUG_TYPE " options");
+
+// Loop unroll and jam factor.
+static llvm::cl::opt<unsigned>
+    clUnrollJamFactor("unroll-jam-factor", llvm::cl::Hidden,
+                      llvm::cl::desc("Use this unroll jam factor for all loops"
+                                     " (default 4)"),
+                      llvm::cl::cat(clOptionsCategory));
+
+namespace {
+/// Loop unroll jam pass. Currently, this just unroll jams the first
+/// outer loop in a Function.
+struct LoopUnrollAndJam : public FunctionPass<LoopUnrollAndJam> {
+  Optional<unsigned> unrollJamFactor;
+  static const unsigned kDefaultUnrollJamFactor = 4;
+
+  explicit LoopUnrollAndJam(Optional<unsigned> unrollJamFactor = None)
+      : unrollJamFactor(unrollJamFactor) {}
+
+  void runOnFunction() override;
+  LogicalResult runOnAffineForOp(AffineForOp forOp);
+};
+} // end anonymous namespace
+
+std::unique_ptr<OpPassBase<FuncOp>>
+mlir::createLoopUnrollAndJamPass(int unrollJamFactor) {
+  return std::make_unique<LoopUnrollAndJam>(
+      unrollJamFactor == -1 ? None : Optional<unsigned>(unrollJamFactor));
+}
+
+void LoopUnrollAndJam::runOnFunction() {
+  // Currently, just the outermost loop from the first loop nest is
+  // unroll-and-jammed by this pass. However, runOnAffineForOp can be called on
+  // any for operation.
+  auto &entryBlock = getFunction().front();
+  if (auto forOp = dyn_cast<AffineForOp>(entryBlock.front()))
+    runOnAffineForOp(forOp);
+}
+
+/// Unroll and jam a 'affine.for' op. Default unroll jam factor is
+/// kDefaultUnrollJamFactor. Return failure if nothing was done.
+LogicalResult LoopUnrollAndJam::runOnAffineForOp(AffineForOp forOp) {
+  // Unroll and jam by the factor that was passed if any.
+  if (unrollJamFactor.hasValue())
+    return loopUnrollJamByFactor(forOp, unrollJamFactor.getValue());
+  // Otherwise, unroll jam by the command-line factor if one was specified.
+  if (clUnrollJamFactor.getNumOccurrences() > 0)
+    return loopUnrollJamByFactor(forOp, clUnrollJamFactor);
+
+  // Unroll and jam by four otherwise.
+  return loopUnrollJamByFactor(forOp, kDefaultUnrollJamFactor);
+}
+
+LogicalResult mlir::loopUnrollJamUpToFactor(AffineForOp forOp,
+                                            uint64_t unrollJamFactor) {
+  Optional<uint64_t> mayBeConstantTripCount = getConstantTripCount(forOp);
+
+  if (mayBeConstantTripCount.hasValue() &&
+      mayBeConstantTripCount.getValue() < unrollJamFactor)
+    return loopUnrollJamByFactor(forOp, mayBeConstantTripCount.getValue());
+  return loopUnrollJamByFactor(forOp, unrollJamFactor);
+}
+
+/// Unrolls and jams this loop by the specified factor.
+LogicalResult mlir::loopUnrollJamByFactor(AffineForOp forOp,
+                                          uint64_t unrollJamFactor) {
+  // Gathers all maximal sub-blocks of operations that do not themselves
+  // include a for op (a operation could have a descendant for op though
+  // in its tree).  Ignore the block terminators.
+  struct JamBlockGatherer {
+    // Store iterators to the first and last op of each sub-block found.
+    std::vector<std::pair<Block::iterator, Block::iterator>> subBlocks;
+
+    // This is a linear time walk.
+    void walk(Operation *op) {
+      for (auto &region : op->getRegions())
+        for (auto &block : region)
+          walk(block);
+    }
+    void walk(Block &block) {
+      for (auto it = block.begin(), e = std::prev(block.end()); it != e;) {
+        auto subBlockStart = it;
+        while (it != e && !isa<AffineForOp>(&*it))
+          ++it;
+        if (it != subBlockStart)
+          subBlocks.push_back({subBlockStart, std::prev(it)});
+        // Process all for insts that appear next.
+        while (it != e && isa<AffineForOp>(&*it))
+          walk(&*it++);
+      }
+    }
+  };
+
+  assert(unrollJamFactor >= 1 && "unroll jam factor should be >= 1");
+
+  if (unrollJamFactor == 1)
+    return promoteIfSingleIteration(forOp);
+
+  if (forOp.getBody()->empty() ||
+      forOp.getBody()->begin() == std::prev(forOp.getBody()->end()))
+    return failure();
+
+  // Loops where both lower and upper bounds are multi-result maps won't be
+  // unrolled (since the trip can't be expressed as an affine function in
+  // general).
+  // TODO(mlir-team): this may not be common, but we could support the case
+  // where the lower bound is a multi-result map and the ub is a single result
+  // one.
+  if (forOp.getLowerBoundMap().getNumResults() != 1)
+    return failure();
+
+  Optional<uint64_t> mayBeConstantTripCount = getConstantTripCount(forOp);
+  // If the trip count is lower than the unroll jam factor, no unroll jam.
+  if (mayBeConstantTripCount.hasValue() &&
+      mayBeConstantTripCount.getValue() < unrollJamFactor)
+    return failure();
+
+  auto *forInst = forOp.getOperation();
+
+  // Gather all sub-blocks to jam upon the loop being unrolled.
+  JamBlockGatherer jbg;
+  jbg.walk(forInst);
+  auto &subBlocks = jbg.subBlocks;
+
+  // Generate the cleanup loop if trip count isn't a multiple of
+  // unrollJamFactor.
+  if (getLargestDivisorOfTripCount(forOp) % unrollJamFactor != 0) {
+    // Insert the cleanup loop right after 'forOp'.
+    OpBuilder builder(forInst->getBlock(), std::next(Block::iterator(forInst)));
+    auto cleanupAffineForOp = cast<AffineForOp>(builder.clone(*forInst));
+    // Adjust the lower bound of the cleanup loop; its upper bound is the same
+    // as the original loop's upper bound.
+    AffineMap cleanupMap;
+    SmallVector<Value, 4> cleanupOperands;
+    getCleanupLoopLowerBound(forOp, unrollJamFactor, &cleanupMap,
+                             &cleanupOperands, builder);
+    cleanupAffineForOp.setLowerBound(cleanupOperands, cleanupMap);
+
+    // Promote the cleanup loop if it has turned into a single iteration loop.
+    promoteIfSingleIteration(cleanupAffineForOp);
+
+    // Adjust the upper bound of the original loop - it will be the same as the
+    // cleanup loop's lower bound. Its lower bound remains unchanged.
+    forOp.setUpperBound(cleanupOperands, cleanupMap);
+  }
+
+  // Scale the step of loop being unroll-jammed by the unroll-jam factor.
+  int64_t step = forOp.getStep();
+  forOp.setStep(step * unrollJamFactor);
+
+  auto forOpIV = forOp.getInductionVar();
+  // Unroll and jam (appends unrollJamFactor - 1 additional copies).
+  for (unsigned i = unrollJamFactor - 1; i >= 1; --i) {
+    // Operand map persists across all sub-blocks.
+    BlockAndValueMapping operandMapping;
+    for (auto &subBlock : subBlocks) {
+      // Builder to insert unroll-jammed bodies. Insert right at the end of
+      // sub-block.
+      OpBuilder builder(subBlock.first->getBlock(), std::next(subBlock.second));
+
+      // If the induction variable is used, create a remapping to the value for
+      // this unrolled instance.
+      if (!forOpIV->use_empty()) {
+        // iv' = iv + i, i = 1 to unrollJamFactor-1.
+        auto d0 = builder.getAffineDimExpr(0);
+        auto bumpMap = AffineMap::get(1, 0, {d0 + i * step});
+        auto ivUnroll =
+            builder.create<AffineApplyOp>(forInst->getLoc(), bumpMap, forOpIV);
+        operandMapping.map(forOpIV, ivUnroll);
+      }
+      // Clone the sub-block being unroll-jammed.
+      for (auto it = subBlock.first; it != std::next(subBlock.second); ++it) {
+        builder.clone(*it, operandMapping);
+      }
+    }
+  }
+
+  // Promote the loop body up if this has turned into a single iteration loop.
+  promoteIfSingleIteration(forOp);
+  return success();
+}
+
+static PassRegistration<LoopUnrollAndJam> pass("affine-loop-unroll-jam",
+                                               "Unroll and jam loops");